A Mathematical Approach to Polymorphic Site Index Curves for Grand Fir

Polymorphic site index curves may be derived for Abies grandis by fitting to stem analysis data the sigmoid function: H = a exp(—b / Ac). This procedure uses the tree as a phytometer to estimate site index. To arrive at a site quality measure closely corresponding to periodic stand increment, the estimation procedure compensates for the effect of site factors that affect the tree only during the seedling and sapling stages of development. Site index is here defined by the annual height increment on a tree 55 feet tall, but expressed as tree height at an index of 50 years. The number of annual rings in 1.5-inch radius from the pith at breast height is used to select the appropriate set of height/age curves from the polymorphic series. Such curves constructed from stem analyses must be applied to a uniform number of the tallest trees per acre at all ages. This number is tentatively set at the rate of 25 trees per acre.